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20                PRACTICAL  ORGANIC  CHEMISTRY

bottle and then replenished with fresh material. Fresh caustic
potash solution is also introduced into the azotometer, unless
the stronger solution is used.

Estimation of Nitrogen^ Second Method. Another method
which dispenses with the small furnace and bicarbonate tube
may also be used. The long" combustion tube is closed at one
end and magnesite in small lumps is introduced into the tube
and shaken down to the closed end until there is a layer of
about 1315 cm. (5  6 in.). This is kept in place by a plug of
asbestos and the tube is filled successively with 5 cm. (2 in.) of
coarse copper oxide, then fine copper oxide mixed with the sub-
stance, a further layer of coarse copper oxide, and finally the

FINE      COARSE
COARSE        CuO _ QUO        CuO  MAGNESITE

I3cM.

ASBESTOS
PLUG

FIG. 17.

copper spiral. The contents of the tube are arranged as shown
in Fig. 17.
The magnesite (MgCO3), which evolves carbon dioxide on
heating, takes the place of the sodium bicarbonate in the
previous method. The air is displaced at the beginning by
heating the magnesite near the closed end of the tube. The
magnesite is again heated towards the end of the combustion to
sweep out the last traces of nitrogen. The disadvantages of
the method are that the magnesite requires to be heated much
more strongly than the sodium bicarbonate before it evolves
carbon dioxide, and the length of the layer of copper oxide is
curtailed.
Kjeldahl's Method.  The organic compound is heated
strongly with sulphuric acid,. which oxidises the organic matter
and converts the nitrogen into ammonium sulphate. The
ammonia is then estimated volumetrically by distilling with
caustic soda and collecting the gas in standard acid. About
0*5 gram of substance is accurately weighed and introduced into
a round Jena flask (500 c.c.), together with 15 c.c. of pure con-